1. Field of the Invention
This invention relates to electronic coordinate converter apparatus, particularly to electronic harmonic oscillator coordinate converters.
2. Description of the Prior Art
In a typical airborne application a system will measure individual angles of a vector with respect to a reference line or coordinate system which may be slightly displaced angularly or off boresight from the desired reference line. The angular displacement may rise from mechanical misalignment which may occur during installation of the system. A vector may be subsequently transformed through the measured angle to another coordinate system using a harmonic oscillator coordinate converter while retaining the angular displacement error.
A harmonic oscillator coordinate converter is used to rotate a vector through an angle from one coordinate system to another. This may occur for example when information such as the direction of a target is provided in geographic coordinates X, Y, Z, and is desired in relation to the sensor coordinates I, J, K, so that the sensor may be directed or pointed in the direction of the target. The transformation of a vector from one coordinate system to another may be accomplished with electronic apparatus described in U.S. Pat. No. 3,473,011, issued to H. Schmid, which does not suggest a means for boresight alignment.
In a moving aircraft the direction from the aircraft to the target is continually changing due to the motion of the aircraft. If a sensor is directed at a target, the pointing direction or vector must be continually updated with a new pointing angle in order to keep the sensor directed at the target while the airplane is moving. In tracking a target, the direction in geographic coordinates X, Y, Z, is continually transformed to the sensor coordinates I, J, K so that the sensor may be continually directed at a target.
In the past, sensor systems were mechanically aligned with a reference line on the aircraft by testing the system for alignment and by making mechanical adjustments. Some sensor systems are now being suspended from a wing of an aircraft in a pod with little or no time for mechanical alignment. The pod is rigidly connected to the wing with two rings extending from the pod and bolted to two ferrules attached to the wing. In this arrangement, a pod may be easily attached or removed from a wing of an airplane, or a pod may be interchanged with another pod in a short amount of time. Two side braces extending from the wing to the pod prevent lateral motion of the pod. However, due to mechanical alignment variations incurred by mounting the pod to the wing, and by mechanical variations between airplanes and pods, the sensor would invariably be misaligned with the reference line of the airplane upon which the other systems would be aligned. The mechanical misalignment was angular in nature and sufficient to require correction either mechanically or electrically. A mechanical correction could be made by unbolting the pod and inserting spacers to align the pod with the reference line. An electrical correction could be made by biasing off the comparator which detects the zero crossing of e.sub.2 (t) during the first rotation period of the harmonic oscillator. The shortcoming of this method is that the modulus (A) of E.sub.2 during this period is not precisely known which caused direct error in the boresight correction angle since the correct scaling of the signal is not known. Another electrical method of correction was to measure the misalignment angle and to transform the vector through the misalignment angle after transforming the vector through the desired angle. The disadvantage of this approach was that since two angles were being serially transformed, the time and error incident to each angle transformation were added together or doubled. The result was that the rate of updating the direction of the sensor to a target was reduced and that the error in directing a sensor to a target was increased.